Method and apparatus for assembling an offshore structure

ABSTRACT

Method and apparatus are disclosed for constructing a buoyantly supported tower at an offshore worksite. Apparatus is disclosed for assembling an elongated tower at an offshore location by connecting a number of tower segments in end-to-end relationship. The apparatus includes a rigid buoyant structure provided with a central vertical well. The buoyant structure is provided with means for locking a tower segment in a preselected position within the well to permit connection of its upper end to an additional tower segment, and for releasing the locked segment to permit the newly connected tower segment to be lowered within the well and locked in place for the connection of an additional tower segment thereto. The method includes the step of positioning a tower segment at a preselected position within the well, aligned with the locked segment and rigidly connected thereto. The locked segment is then released and the rigidly connected segments are lowered and locked into place to permit the joining of additional tower segments. Additional segments are joined to the assembled section and lowered toward bottom until the tower extends to bottom. The buoyant assembly may then be elevated relative to the tower and rigidly affixed to the upper end.

8 United States Patent 1 [111 3,736,756 Lloyd June 5, 1973 I54] METHOD AND APPARATUS FOR [57] ABSTRACT AN OFFSHORE Method and apparatus are disclosed for constructing a buoyantly supported tower at an offshore worksite. [75] Inventor: James R. Lloyd, Houston,Tex. Apparatus is disclosed for assembling an elongated tower at an offshore location by connecting a number [73] Asslgnee' 3 t Pr?rductlon Research of tower segments in end-to-end relationship. The apous paratus includes a rigid buoyant structure provided [22] Filed: Nov. 3, 1971 with a central vertical well. The buoyant structure is provided with means for locking a tower segment in a [21] Appl' 195087 preselected position within the well to permit connection of its upper end to an additional tower segment, [52] U.S.Cl. ..61/46.5 and for releasing the locked segment to permit the [51] Int. Cl. ..E02b 17/00 newly connected tower segment to be lowered within [58] Field of Search ..6l/46.5, 46, 63, the well and locked in place for the connection of an 61/50 additional tower segment thereto. The method includes the step of positioning a tower segment at a [56] References Cited preselected position within the well, aligned with the locked segment and rigidly connected thereto. The UNITED STATES PATENTS locked segment is then released and the rigidly con- 3,031,997 5/1962 Nesbitt ..6l/46.5 x nected segmentsare lowered and locked into place to 1,758,606 5/1930 Jacobs ..6l/46 permit the joining of additional tower segments. Addi- 2,365,179 12/1958 McNeill l/ 6-5 tional segments are joined to the assembled section 3,643,446 2/1972 and lowered toward bottom until the tower extends to 2,908,142 10/1959 Suderow ..6l/46.5 bottom The buoyant assembly may then be elevated I relative to the tower and rigidly affixed to the upper Primary Examiner-Jacob Shapiro emi Attorney-James A. Reilly, John B. Davidson, Lewis H. Eatherton et al. 9 Claims, 16 Drawing Figures 1; W- M, l T 77 l5 1 l3 l9 29 PAH-.NTEUJUR: I975 SHEEI 1 (1F 6 JAMES R LLOYD I N VEN TOR.

ATTORNEY PATENTEUJUA: 5 197a SHEET 2 OF 6 INVENTOR.

D Y O L L R S E M m ATTORNEY PATENTEUJUH 5 I975 3,736. 756 SHEET 3 OF 6 Hun-w m 93 f I; w

\ I sl JAMES R. LLOYD 95 INVENTOR ATTORNEY PATENTEUJUH 5 I975 SHEET 0F 6 JAMES R. LLOYD INVENTOR.

PATENTEUJUH 51975 SHEET 5 [1F 6 JAMES R. LLOYD INVENTOR.

ATTORNEY k JAMES R. LLOYD INVENTOR.

Y Q 4.. F4 LL ATTORNEY BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a buoyantly supported tower constructed of a plurality of segments connected in end-to-end relationship and to apparatus and method for installing the same at an offshore location.

2. Description of the Prior Art As efforts directed toward exploration for and production of crude oil and natural gas are extended into deeper waters, depths will ultimately be reached which will make impractical the rigid, bottom-founded platforms presently employed to support such operations. One alternative for deep water operations is a buoyantly supported tower which extends from the water surface to the ocean bottom. Such structures normally include an elongated tower having at least one buoyancy chamber near the upper end and a pivoted anchor assembly which permits the tower to sway with respect to the submerged bottom and thereby alleviate bending stresses. The anchoring assembly may, for example, include a pile-founded base in combination with a universal joint.

Because the economic advantages offered by buoyantly supported towers are peculiar to deep water locations, the tower structures are normally of great length and may, for example, be as long as 1000 feet or more. Fabrication and installation of these long, rigid towers present a number of difficulties. Construction of the tower on shore may be economically unattractive becauseof lack of facilities capable of accommodating a structure of such extreme length. Additional problems are posed by the necessity of transporting the tower from the land fabrication site and launching it into the water. It has been proposed to use a graving yard to overcome these problems. A graving yard offers certain advantages since the tower can be constructed at a dry land facility and the floated out after the graving yard is flooded. This approach also has distinct disadvantages. Mobilization of a graving yard requires acquisition of a coastal property side, excavation of a construction area large enough for the tower and the assembly of a great deal of heavy equipment at the site which is necessary for fabrication of the tower. The cost of mobilizing a graving yard therefore requires substantial outlays as a prerequisite to construction and flotation of the tower.

Regardless of how fabrication and flotation are ac complished, prior art installation techniques suffer from additional difficulties stemming from the necessity of floating the long, slender tower to the installation site and upending it to place it in the desired vertical configuration. During the period the tower is floating horizontally in the water, it will be subjected to significant bending stresses. The stresses induced in the tower during flotation and installation will frequently exceed those which will be imposed on the tower following installation, requiring a stronger and therefore more expensive tower structure than otherwise might be required. It will therefore be apparent that there exists a need for an improved method and apparatus for fabrication and installation of buoyantly supported towers for use in deep water.

SUMMARY OF THE INVENTION The method and apparatus of the present invention alleviate the problems outlined above and take advantage of the unique features of buoyantly supported towers to permit their assembly and installation at deep water sites at a substantially reduced cost. In accordance with the present invention, apparatus is provided for assembling an elongated tower at an offshore location by connecting a plurality of tower segments in endto-end relationship. The tower segments may conveniently be fabricated at an onshore site. The apparatus includes a rigid buoyant structure provided with a central vertical well extending therethrough. The structure further includes means for locking a tower segment in a preselected position-within the well to permit the connection of an additional tower segment to its upper end. The locked segment may be released to permit the newly connected tower segment to be lowered into the preselected position within the vertical well and locked into place so an additional tower segment may be added. The assembled tower further includes a means connected near the lower end of the string of tower segments for pivotally anchoring the same to a submerged bottom.

The method of the invention includes the steps of positioning a tower segment at a preselected position within the well and locking it to the buoyant structure. An additional segment is then positioned within the well, aligned with the locked segment and rigidly connected thereto. The locked segment is then released and the newly joined segments are lowered and locked within the well to permit the connection of another segment. Additional segments are rigidly connected and lowered until the tower extends to bottom. The lower end of the tower is then pivotally anchored to bottom to permit the tower to sway in response to environment forces. A work platform may then be affixed to the upper end of the buoyant structure and the structure elevated with respect to the tower, as for example by deballasting.

The apparatus and method of the present invention permit buoyantly supported towers to be assembled offshore and installed in an upright configuration. The apparatus and method of the present invention thus eliminate the difficulties inherent in fabricating these elongated structures onshore, floating them to the installation site and upending them and therefore offer significant advantages over those available heretofore,.

BRIEF DESCRIPTION OF THE DRAWINGS line 4'4 of FIG. 3.

FIG. 5 is a cross-sectional elevation view of the apparatus of FIG. 3 taken along line 5-5.

FIG. 6 depicts the apparatus of FIG. 4 further including a telescoping support apparatus for controlling descent of tower segments supported within the well.

FIG. 7 is an elevation view of the lowermost segment of the assembled tower in combination with a pivot assembly and a pile-founded base.

FIG. 8 is a plan view of the apparatus of FIG. 7.

FIG. 9 is an elevation view of another embodiment of the pivotally anchored base of the apparatus of the invention.

FIG. 10 is a plan view of the apparatus of FIG. 9.

' FIG. 11 is a series of schematic elevation views A-F depicting sequentially the installation of the buoyantly supported tower of the present invention in accordance with the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With respect to FIGS. 1 and 2 of the drawings, in which identical numerals have been used to designate identical structural elements, a rigid buoyant structure is shown which is used to support and to provide stability to tower segments during assembly as well as to support and stabilize the assembled tower. The rigid buoyant structure is designated generally by numeral lll and is shown constructed of three vertical buoyancy chambers l3 rigidly connected to one another by cross members 15 so as to leave a vertical well 17 extending through the center of the buoyant structure. The well may be open on one side to permit flotation of tower segments thereinto. The buoyancy chambers are each provided with ports and associated equipment not shown in the drawings for introducing and withdrawing ballast whereby the buoyancy and draft of the buoyant structure can be controlled. The structure shown has a triangular cross section. This particular shape has the advantage that it will maintain its cross-sectional configuration without requiring any internal framing members which might obstruct the vertical well. It will be apparent, however, that other configurations of buoyancy chambers and structural members are suitable for use.

Guide structures 19 are shown affixed to the buoyant structure around the periphery of the well and serve to guide tower segments as they are lowered through the well. A flared, funnel-like end assembly 21 may conveniently be. attached to the upper end of each such guide structure to aid in introducing the tower segments into the guide structure as it is lowered into the well by a derrick barge or the like. As shown, the guide structures are configured as segments of right circular cylinders which are open to the well to permit tower segments to pass therethrough. The guide structures are shown spaced vertically within the well in the buoyant structure and in axial alignment with one another. Clearance between the tower segments and the guide structures will preferably be in the neighborhood of 1 inch or less, though somewhat greater tolerances are permissible. It will be apparent that continuous guide structure can also be used and that configurations of guide structures other than those shown are suitable for achieving the desired result. The inner surface of the well can for example be employed, or if the crosssectional area of the tower is to be varied an adjustable guide structure can be used.

It is preferred that tower segments be guided as they are lowered through the buoyant structure to .increase stability. When guided by the structure, any time the assembled towersegments begin to tilt from a vertical orientation, they immediately come into contact with the guide structures and thereby buoyant assembly ll. This has the effect of retarding tilting since whenever the buoyant structure is tilted, a righting moment will be generated by the buoyancy chambers and will tend to restore the entire structure to a vertical orientation. Guiding the tower segments through the buoyant structure thereby imparts a high degree of stability to the tower assembly permitting its installation in the presence of wind, waves, currents and other environmental forces that tend to displace the assembled tower from vertical.

A number of tower segments depicted generally by numeral 23 are shown in FIGS. 1 and 2. These segments are shown positioned within vertical well 17 and are each comprised of three vertical structural members 25 which may, for example, be tubular members held in rigid relation to one another by appropriate cross members 27. Use of tubular members has the advantage that if a specific diameter-to-thickness ratio is used, e.g., for steel a ratio of about thirty, and the members are maintained water tight, the resultant structure will be neutrally buoyant. This substantially reduces the buoyancy and lifting requirements during installation. The cross-sectional configuration of each tower segment may conveniently be geometrically similar to that of the buoyant structure. In any event, each tower segment should preferably have vertical members, as for example, vertical structural members 25, situated about its perimeter such that the vertical members will be received within guide structures 19 consistent with the tolerances specified above.

Conductor guides 29 are shown rigidly affixed to the tower segments at selected positions around their periphery. These may conveniently be affixed to cross members 27 as shown in the drawing. The conductor guides will normally be situated around the perimeter of the tower segmentsat each location where a well conductor is to be positioned. The conductor guides may be circular cylindrical structural elements as shown and should be sized to receive and to guide a casing string or similar conductor pipe therethrough. Each conductor guide may have a flared section 31 at the upper end to assist in leading the conductor pipe into the guide assembly. Besides those used for well conductors, additional vertically aligned conductor guides may be provided for such other conduits as may be required between the surface and the lower end of the assembled buoyantly supported tower. These can, for example, include utility conduits to introduce ballast material into a ballast chamber near the lower end of the tower, to introduce a lubricating fluid into a housing surrounding the pivot assembly, or to introduce ballast material into the base of the tower.

The upper and lower ends of the vertical structural members 25 which form a part of tower segments 23 preferably have alignment assemblies affixed thereto. The tops may, accordingly, be conical or frusto-conical as depicted by numeral 33 of FIGS. 4 and 5. The lower ends of the structural members may also have conical or, as is shown, frusto-conical bottoms 35 which are directed oppositely from those mounted on the upper ends so that the tops and bottoms are nestable. The use of tapered, nestable end members permits the ends of the tower segments to be aligned more readily with one another.

FIGS. 3, 4 and 5 depict in detail means for anchoring tower segments 23 within vertical well 17 in buoyant structure 11. As shown in FIG. 5, the releasable interengaging or locking means for rigidly locking the tower segments to the buoyant structure is a hydraulic locking pin assembly, although other types of hydraulically actuated systems as well as mechanical or electrical systems may also readily be employed. Hydraulic cylinder housing 37 having a cylindrical chamber 39 therein and an aperture 41 in one end is shown attached to one of the guide structures 19 which will normally be positioned near the upper end of buoyant structure 11. The guide structure shown has a corresponding aperture therethrough designated by numeral 43 and the hydraulic cylinder housing is attached to the guide structure such that the apertures in each are aligned. A bearing reinforcement plate 45 with corresponding aperture 47 may, as is shown in the drawing, be placed between the hydraulic housing and guide structure to increase the area of the bearing surface. A piston 49 is slidable within chamber 39 and is connected to locking pin 51 which is aligned with and extensible through the aperture in the end of the hydraulic housing. Sealing members and hydraulic lines as may be required are also provided in combination with the piston and hydraulic cylinder.

To accommodate the hydraulically actuated locking pin, each tower segment is provided with an aperture 53 which may be positioned near the upper end of at least one of the vertical structural members 25 and should be aligned so as to be in registry with aperture 43 in guide structure 19 when the segment is at a preselected position within the buoyant assembly. Aperture 53 in the tower segment may have beveled edges, as shown, to aid in guiding the extensible locking pin thereinto. A bearing reinforcement plate 55 having an aperture 81 therethrough aligned with aperture 53 in the tower segment may be rigidly affixed to the vertical member of the tower segment to be received within the guide structure if it is desired to increase the area of the bearing surface.

Whenever the upper end of a tower segment is at the preselected fabrication elevation, i.e., the level where it is to be joined to the next tower segment to be added, the aperture in the tower segment should be aligned with that in the guide structure. The hydraulic locking pin assembly may then be activated by introducing hydraulic fluid into cylinder 39 to extend locking pin 51 into aperture 53 and thereby lock the tower segment to the buoyant structure. With the segment secured within the well the buoyant structure is ready to receive an ad ditional tower segment. The additional segment may be positioned by guidingly lowering the same into well 17.

until it comes to rest atop the anchored tower segment and is thus supported by the buoyant structure.

While normally, as pointed out above, each additional tower segment is guidingly lowered so as to be in engagement with and supported by those segments already rigidly anchored to the buoyant structure, whereby the additional segment is supported by the buoyant structure, in some cases it may be desirable to maintain close control of the lowering of the additional segment over the last few feet separating the segments. FIG. 6 depicts a telescoping support apparatus which may optionally be employed and will permit close control of the lowering of the additional towerYsegment.

The telescoping support means may include a hydraulic cylinder as depicted by numeral 59 having an extensible piston 61 with a dog, as for example cantilever support shoe 63, on its upper end. The hydraulic cylinder is rigidly affixed to buoyant structure 11 by structural support 65. The cantilever support shoe is rotatable to permit it to be extended into or to be retracted from the path of each tower segment as it is guided within the buoyant structure. Tower segments may be adapted for use with the telescoping support by means ofa slot 67 positioned near the lower end of vertical structural member 25. The slot should be aligned so as to be in registry with cantilever support shoe 63 when the tower segment is at a preselected elevation within the well. It should further be sized to receive cantilever support shoe 63 and may include bearing reinforcement material 69 to reduce unit stresses at the support surfaces. After the additional tower segment has been guidingly lowered into the center well and is at the desired elevation, support shoe 63 is rotated and inserted into slot 67 and hydraulic cylinder 59 is actuated to extend piston 61 until the weight of the additional tower segment is transferred to the buoyant structure. Thereafter, hydraulic pressure may be gradually bled off hydraulic cylinder 59 so that piston 61 slowly retracts and the tower segment descends through the guide structures until the clearance between the upper end of the assembled section of tower segments anchored within the well and the lower end of the additional segment to be connected thereto is that desired for rigidly affixing them to one another. While the telescoping support means has been described in relation to only one of the vertical structural members which make up each of the tower segments, it will be understood that a number of these assemblies would normally be employed to support and control the vertical movement of the additional tower segment. Other telescoping support means which similarly have the advantage of permitting close control over relative motion between tower segments as the nestable end members approach one another and of permitting connection of the segments without the weight of the additional tower segment acting on the surfaces to be joined will readily be apparent to those skilled in the art.

Rigid connection of adjacent tower segments in an end-to-end relationship is normally accomplished by welding. While welded rigid connections are preferred, other means of rigidly affixing tower segments in endto-end relationship, for example by mechanical means such as by pins or the like, will be clear to those skilled in the art. FIG. 4 depicts two spaced guide structures 19 affixed to buoyant structure 11. These guide structures are situated near the upper end of the buoyant structure and the space therebetween coincides with the fabrication elevation. As will be seen in FIG. 4, the vertical structural members 25 to be welded together to join the tower segments are accessible between the spaced structures whereby their connection is facilitated. The weld station is normally positioned near the upper end of the buoyant structure so that when the tower segments each occupy the preferred position within the well, the fabrication elevation is both above the water surface and the zone subjected to wave action. The lower tower segment in FIG. 4 is shown locked to the buoyant structure by means of the hydraulically actuated locking pin assembly discussed above. Engagement of this anchor assembly rigidly affixes the lower tower segment and any additional tower segments connected to it to the buoyant structure so that the assembled tower section is supported by the buoyant structure. The upper tower segment is shown in FIG. 4 positioned within the well after it has been lowered through the guide structures into alignment with the lower segment. The upper segment is resting on the rigidly anchored lower segment and is thus supported by the buoyant structure. Weld material 71 is shown distributed around the juncture of the frustoconical top 33 of the lower tower segment and the conical bottom 35 of the upper segment forming a rigid connection therebetween.

With the additional segment rigidly connected to the section of assembled tower segments supported by the buoyant structure the entire assembled tower structure may be released and lowered. A means for lowering tower segments while supporting them from the buoyant structure is shown in FIGS. 1 and 2 and includes a winch 73 shown mounted atop the buoyant structure on one of the buoyancy chambers 13. The winch operates through a fixed block 75 mounted atop one of the cross members 15 and a traveling block 77 suspended below it. While only one winch is shown it will be understood that several would normally be used. The primary function of the winch system is to support and lower the assembled tower segments within the well after each additional segment is attached to the upper end of the string whereby the hydraulically actuated locking pin can be released by disengaging it. Thereafter, the winch system is employed to lower the assembled segments towards bottom, preferably through guide structures within the well, while supporting same with the buoyant assembly. When the upper end of the string reaches the preselected fabrication elevation, the hydraulic locking pin assembly is re-engaged and the assembled section is locked into place in preparation for connection of an additional tower segment.

The means for anchoring the lower end of the fully assembled tower to a submerged bottom while permitting it to sway in response to environmental forces may include a pivot assembly and a base provided with means for anchoring it to a submerged bottom. FIGS. 7 and 8 depict one configuration of the bottom tower segment 23 of the buoyant tower of the present invention provided with a pivot assembly. Shown also is a pile-founded base 93. The bottom tower segment is shown having a ballast chamber 79 mounted therein and provided with a ballast fill conduit 81. The ballast conduit extends to the upper end of the tower through vertically aligned conductor guides 29 depicted in FIGS. .1 and 2. A pivot assembly shown as universal joint 83 is attached to the lower end of the tower segment. The pivot may suitably be enclosed within a housing such as is designated by numeral 85 to permit it to be immersed-in a bath ofa lubricating fluid. Guide base assembly 87 is shown attached to the lower end of the pivot assembly and includes three guide funnels 89 spaced around its perimeter. An anchor post receiving housing 91 having an aperture 92 for receiving a locking pin is also shown mounted on the guide assembly. Base 93 is depicted anchored to submerged bottom 95 by means of pilings 97. The base is provided with pile sleeves 99 having funnel-shaped upper ends 101 to assist in leading the pilings thereinto. Upright guide posts 103 are spaced about the perimeter of the base and are configured and oriented such that they will mate with guide funnels 89 when the guide base assembly 87 is lowered into engagement with base 93. Anchor post 105 extends upwardly from base 93 and includes an aperture 107 to receive a locking pin whereby the anchor post can be locked into the anchor post receiving housing 91, thereby locking guide base 87 to base 93 whereby the tower is pivotally anchored to the base. While the alignment and mating of the guide base assembly and base in the embodiment shown may be accomplished by carefully positioning the buoyant structure over the base and, if required, employing a submersible vehicle with a manipulator, the use of guide lines to guide structures into registry with a submerged base is well known in the art and such a system may readily by employed to accomplish the alignment of the apparatus shown.

FIGS. 9 and 10 depict another embodiment of apparatus for use in anchoring the tower to a submerged bottom while permitting it to sway. Shown in FIGS. 9 and 10 is another configuration of the lowermost tower segment 23 of the buoyantly supported tower of the invention. The segment shown is provided with a ballast tank 109 which is configured to form a pivot attached to the lower end thereof for anchoring the tower to submerge bottom 95. The pivotal ballast tank is shown configured as a right circular cylinder having a hemispherical lower end and is provided with a ballast fill line 111 which extends towards the upper end of the tower through conductor guides as shown in FIGS. 1 and 2. In this embodiment the tower is anchored by filling the pivotal ballast tank with ballast material. Because of the configuration of the lower end of the pivotal ballast tank, the tank is free to pivot with respect to bottom whereby the tower will sway in response to environmental forces while anchored by the ballast material.

Installation of the apparatus'of the invention is depicted sequentially in FIGS. 11A through lllF. In FIG. 11A the rigid buoyant structure 11 is depicted floating in a body of water 113. A base 93 is shown anchored to the submerged bottom by means of pilings as is shown in detail in FIGS. 7 and 8. Although not shown in the drawings, a work boat will normally be positioned on the water surface adjacent the rigid buoyant structure and provided with a derrick depicted by numeral 115 for handling segments of the tower assembly. The'derrick on the work boat has picked up the first tower segment 23 which is shown as including a ballast chamber, pivot assembly and guide base 87, such as those shown in detail in FIGS. 7 and 8, attached to its lower end. The derrick has transferred the segment into position over buoyant structure 11 prior to lowering it into the well extending through the buoyant structure. It will be noted that alternatively rigid buoyant structure 11 may be floated to the installation site with the first segment, pivot assembly and guide assembly already anchored within the inner well. This permits the guide assembly to be of larger diameter than the well through the buoyant structure. It will further be noted that base 93 may also be attached to the lower end, thereby obviating the use of guide base 87 and associated equipment employed to align the assembled tower with a preinstalled base.

FIG. 11B shows the apparatus of FIG. 11A after the first tower segment has been positioned with the well extending through the buoyant structure by lowering the guiding the same therewithin. The position of the segment within the well has been adjusted by means of the derrick, as required to place the upper end at the fabrication elevation, and the segment has been locked to the buoyant structure, as for example by means of the hydraulic anchor assembly discussed above in relation to FIG. of the drawings. With the tower segment anchored within the well and supported by the buoyant structure the derrick on the work boat is no longer required and may be employed to transfer another tower segment to the structure and lower it into the well therewithin.

FIG. 11C depicts the apparatus of FIG. 118 after an additional tower segment 23 has been lowered by the derrick and guided into the well. The additional segment is in alignment with and resting atop the first segment already rigidly affixed within the well. In this position the weight of the additional tower segment is carried by the buoyant assembly, thereby freeing the derrick to position another tower segment within the well. The two tower segments may then be rigidly joined by welding or the like and the joined segments lowered guidingly within the well by means of a winch on the buoyant structure. While it is preferred to lower the assembled section by means of a hoist on the buoyant structure, it will be apparent this may be accomplished by a derrick barge or the like. When the joined segments have been lowered a sufficient distance such that the top of the assembled section of tower segments is at a preselected fabrication elevation, the joined segments are secured to the buoyant structure in preparation for the connection of an additional tower segment. Thus, as shown in FIG. 11D, two tower segments have been rigidly joined together and lowered to position the upper end of the unit at the preselected fabrication elevation and a third segment has been guided into position for joining. This procedure is continued until, as shown in FIG. 11E, sufficient tower segments have been guided into, joined and lowered from the buoyant structure to extend the assembled section of tower segments from the buoyant structure to the base. Guide base 87 is then aligned with and locked to base 93 thereby anchoring the tower to the submerged bottom while permitting it to sway in response to environmental forces.

With the assembled tower pivotally anchored to bottom a work platform may be assembled atop the tower. Thus, as shown in FIG. 11E a series of platform sections may be affixed to the upper end of the buoyant structure to form a work platform atop the tower. FIG. 11F depicts the structure after buoyant structure 11 has been elevated, as for example by deballasting it, to raise the platform deck to a height well above the level of wave action. A drilling rig 119 is shown positioned -on the platform deck of the completed structure. Buoyant structure 11 is then rigidly affixed to the upper end of the assembled tower by anchoring it to a tower segment or by welding it or mechanically securing it thereto. The elevation of the buoyant structure should be such that the buoyancy chambers remain at least partially submerged and will generate a righting moment whenever the tower makes an excursion from vertical, which will stabilize the tower and restore it to a vertical orientation.

It will be apparent that while the work platform has been described as affixed to the buoyant structure, which is subsequently elevated, it may instead be affixed to an upward extension of the tower thereby diminishing the necessity of elevating the buoyant structure. It will further be apparent that the work platform may be affixed to the upper tower segment and the buoyant structure then lowered, as by pulling it down with cables or the like, prior to its being rigidly affixed to the assembled tower.

It will be noted that the discussion of the installation of the buoyantly supported tower of the invention has been limited to a tower having a pivot assembly and base like those depicted in FIGS. 7 and 8. If the apparatus of FIGS. 9 and 10 is employed instead, the procedure for installation does not vary substantially from that described with respect to FIG. 11. The primary distinction is that the base is an integral part of the lowermost tower segment whereby guiding the assembled tower into alignment with a preinstalled base is no longer necessary.

It will be apparent from the foregoing that the method and apparatus of the invention make it possible to assemble a buoyantly supported tower at an offshore installation site thereby eliminating difficulties encountered in fabricating and launching these structures from locations onshore, in floating towers to location, and upending them to place them in a vertical orientation and at the same time making possible substantial cost savings. The method and apparatus of the present invention therefore have numerous advantages over prior art buoyantly supported towers and their assembly and installation techniques.

What is claimed is:

l. A method of assembling in a body of water an elongated tower comprised of a plurality of tower segments for connection in end-to-end relation comprismg:

a. situating a rigid buoyant structure having a central, vertical well extending therethrough at the surface of the body of water;

b. positioning a first tower segment at a preselected position within said well;

c. locking said first segment to said rigid buoyant structure;

d. positioning a first additional tower segment within said well and aligning the same with said locked first segment;

e. rigidly connecting said first additional tower segment to said locked first segment;

f. releasing said locked first segment, lowering said rigidly connected segments and locking the same to said buoyant structure at a preselected position within said well to permit a-second additional tower segment to be positioned within said well and connected to the upper end of said rigidly connected segments;

g. continuing to position additional tower segments within said well, rigidly connect said segments to the assembled tower formed by the rigidly connected tower segments and lower said assembled towerto permit additional segments to be connected thereto until the assembled tower extends to the bottom beneath said body of water;

h. anchoring the lower end of said assembled tower to said submerged bottom, and

i. affixing said rigid buoyant structure to said assembled tower near the upper end thereof.

2. The method of claim 1 further comprising the step of elevating said rigid buoyant structure with respect to said assembled tower prior to affixing said rigid structure thereto.

3. The method of claim 2 wherein said rigid buoyant structure is elevated by deballasting the same.

4. Apparatus comprising a rigid buoyant structure adapted to float in a body of water and provided with a central, vertical well extending therethrough and with releasable locking means for locking a tower segment in a preselected position within said well to enable the connection of an additional tower segment to the upper end thereof in combination with a plurality of tower segments rigidly connected in end-to-end relationship, said tower segments extending downwardly from said rigid buoyant structure to the bottom of said body of water, the upper end of said assembled segments being rigidly affixed to said buoyant structure and the lower end being provided with means for anchoring the same to said bottom.

5. Apparatus defined by claim 4 wherein the crosssection of each said tower segment is geometrically similar to that of the well extending through said buoyant structure.

6. Apparatus defined by claim 4 wherein said means for anchoring the lower end of said tower to the submerged bottom includes a base provided with means for anchoring same to the submerged bottom and a pivot assembly for interconnecting said tower and said base.

7. Apparatus defined by claim 4 wherein said means for anchoring the lower end of said tower to the submerged bottom includes a pivotal ballast tank rigidly affixed to the lower end of the string of rigidly connected tower segments.

8. Apparatus as defined by claim 4 wherein the lowermost tower segment includes a ballast chamber and has a pivot assembly and guide base attached to the lower end thereof.

9. Apparatus as defined by claim 7 further including ballast conduit extending upwardly from said ballast chamber to a point near the upper end of the tower. 

1. A method of assembling in a body of water an elongated tower comprised of a plurality of tower segments for connection in endto-end relation comprising: a. situating a rigid buoyant structure having a central, vertical well extending therethrough at the surface of the body of water; b. positioning a first tower segment at a preselected position within said well; c. locking said first segment to said rigid buoyant structure; d. positioning a first additional tower segment within said well and aligning the same with said locked first segment; e. rigidly connecting said first additional tower segment to said locked first segment; f. releasing said locked first segment, lowering said rigidly connected segments and locking the same to said buoyant structure at a preselected position within said well to permit a second additional tower segment to be positioned within said well and connected to the upper end of said rigidly connected segments; g. continuing to position additional tower segments within said well, rigidly connect said segments to the assembled tower formed by the rigidly connected tower segments and lower said assembled tower to permit additional segments to be connected thereto until the assembled tower extends to the bottom beneath said body of water; h. anchoring the lower end of said assembled tower to said submerged bottom, and i. affixing said rigid buoyant structure to said assembled tower near the upper end thereof.
 2. The method of claim 1 further comprising the step of elevating said rigid buoyant structure with respect to said assembled tower prior to affixing said rigid structure thereto.
 3. The method of claim 2 wherein said rigid buoyant structure is elevated by deballasting the same.
 4. Apparatus comprising a rigid buoyant structure adapted to float in a body of water and provided with a central, vertical well extending therethrough and with releasable locking means for locking a tower segmEnt in a preselected position within said well to enable the connection of an additional tower segment to the upper end thereof in combination with a plurality of tower segments rigidly connected in end-to-end relationship, said tower segments extending downwardly from said rigid buoyant structure to the bottom of said body of water, the upper end of said assembled segments being rigidly affixed to said buoyant structure and the lower end being provided with means for anchoring the same to said bottom.
 5. Apparatus defined by claim 4 wherein the cross-section of each said tower segment is geometrically similar to that of the well extending through said buoyant structure.
 6. Apparatus defined by claim 4 wherein said means for anchoring the lower end of said tower to the submerged bottom includes a base provided with means for anchoring same to the submerged bottom and a pivot assembly for interconnecting said tower and said base.
 7. Apparatus defined by claim 4 wherein said means for anchoring the lower end of said tower to the submerged bottom includes a pivotal ballast tank rigidly affixed to the lower end of the string of rigidly connected tower segments.
 8. Apparatus as defined by claim 4 wherein the lowermost tower segment includes a ballast chamber and has a pivot assembly and guide base attached to the lower end thereof.
 9. Apparatus as defined by claim 7 further including ballast conduit extending upwardly from said ballast chamber to a point near the upper end of the tower. 